Objective optical system for endoscope

Abstract

An objective optical system for endoscope includes, in order from an object side: a first lens having a negative refractive power; a second meniscus lens having a positive refractive power with a convex surface thereof directed toward the object side; an aperture stop; a third lens having a biconvex shape, and a cemented lens having a positive refractive power as a whole, in which a fourth lens having a biconvex shape and a fifth lens having a negative refractive power are cemented.

What is claimed is: 
     
       1. An objective optical system for endoscope, comprising in order from an object side:
 a first lens having a negative refractive power; 
 a second meniscus lens having a positive refractive power with a convex surface thereof directed toward the object side; 
 an aperture stop; 
 a third lens having a biconvex shape, and 
 a cemented lens having a positive refractive power as a whole, in which a fourth lens having a biconvex shape and a fifth lens having a negative refractive power are cemented, wherein 
 the objective optical system for endoscope satisfies the following conditional expressions (1), (2), and (9)
   −3≤ f   1   /Ih≤− 1.2  (1)
 
   0.25≤ L   3   _   5   /L≤ 0.7  (2)
 
   0.2≤ r   2   /r   3 ≤0.8  (9)
 
 
 where, 
 f 1  denotes a focal length of the first lens; 
 Ih denotes the maximum image height of the objective optical system for endoscope; 
 L 3   _   5  denotes a distance along an optical axis from a surface on the object side of the third lens up to a surface on an image side of the fifth lens, 
 L denotes a distance along the optical axis from a surface on the object side of the first lens up to an image plane, 
 r 2  denotes a radius of curvature of a surface on an image side of the first lens, and 
 r 3  denotes a radius of curvature of a surface on the object side of the second meniscus lens. 
 
     
     
       2. The objective optical system for endoscope according to  claim 1 , wherein the objective optical system for endoscope satisfies the following conditional expressions (3) and (4)
   0.5≤ d   5i   /d   23 ≤10  (3)
 
   3≤ L/d   23 ≤100  (4)
 
 where, 
 d 5i  denotes a distance along the optical axis from a surface on the image side of the fifth lens up to the image plane, 
 d 23  denotes a distance on the optical axis from a surface on the image side of the second meniscus lens up to the surface on the object side of the third lens, and 
 L denotes a distance along the optical axis from the surface on the object side of the first lens up to the image plane. 
 
     
     
       3. The objective optical system for endoscope according to  claim 1 , wherein the first lens has a planoconcave shape with a flat surface directed toward the object side. 
     
     
       4. The objective optical system for endoscope according to  claim 1 , wherein the objective optical system for endoscope satisfies the following conditional expression (5)
   7≤ f   2   /Ih≤ 200  (5)
 
 where, 
 f 2  denotes a focal length of the second meniscus lens, and 
 Ih denotes the maximum image height of the objective optical system for endoscope. 
 
     
     
       5. The objective optical system for endoscope according to  claim 1 , wherein the objective optical system for endoscope satisfies the following conditional expression (6)
   −2≤ f   12   /f   345 ≤−0.55  (6)
 
 where, 
 f 12  denotes a combined focal length of lenses from the first lens up to the second meniscus lens, and 
 f 345  denotes a combined focal length of lenses from the third lens up to the fifth lens. 
 
     
     
       6. The objective optical system for endoscope according to  claim 1 , wherein the objective optical system for endoscope satisfies the following conditional expression (7)
   2≤ f   2   /f   3 ≤100  (7)
 
 where, 
 f 2  denotes a focal length of the second meniscus lens, and 
 f 3  denotes a focal length of the third lens. 
 
     
     
       7. The objective optical system for endoscope according to  claim 1 , wherein the objective optical system for endoscope satisfies the following conditional expression (8)
   0.4≤ f   1   /f   5 ≤2  (8)
 
 where, 
 f 1  denotes the focal length of the first lens, and 
 f 5  denotes a focal length of the fifth lens. 
 
     
     
       8. The objective optical system for endoscope according to  claim 1 , wherein the objective optical system for endoscope satisfies the following conditional expression (10)
   2≤ d   3   /d   34 ≤15  (10)
 
 where, 
 d 3  denotes a lens thickness of the third lens, and 
 d 34  denotes a distance along the optical axis from a surface on an image side of the third lens up to a surface on an object side of the fourth lens.